AMPLIFYING YOUR ACOUSTIC GUITAR WITHOUT
FEEDBACK

Copyright 2007 Steve Carmody

Manufacturers of pickups for acoustic guitars are at the top of their game right now. I
don’t know of any pickup systems currently made by major manufacturers that aren’t
high quality. But still, getting a good live amplified acoustic guitar sound often remains
elusive for many players. One issue is getting enough volume without feedback. Having
a good pickup system and an amplifier designed specifically for acoustic guitars are
absolutely essential to getting a good sound, but often not sufficient to avoid feedback
problems.

There are a number of factors I can think of which can contribute to feedback problems,
and a couple of additional solutions to suggest if all else fails.

1. THE ROOM

2. OVERALL VOLUME

3. PRE-AMP SETTINGS

4. MECHANICAL ISSUES WITH THE GUITAR

5. ADDITIONAL SOLUTIONS

THE ROOM

The setting in which you are playing impacts on your ability to increase the volume of
your guitar without getting feedback. A small room can certainly be an issue. The signal
coming from your amp can bounce off walls and windows, head right back into the box
of your guitar and trigger feedback. If you are playing a small room, set the amp and all
speakers such that the path back to the guitar is as complicated as possible. Think of a
pool table. If you shoot the cue ball straight at a rail – it bounces right back. But if you
shoot the ball at the correct angle, it may never have the energy to return where it started.
With this in mind, you should orient your guitar amp and any monitor speakers so that
they point to your ears, and not the soundhole of the guitar. Any reflective surfaces, bare
walls, and especially windows, can be a problem, so if you control the room, padding the
walls with blankets,foam, even egg cartons, will do wonders; but if you show
up at a gig and the room is small – think angles

OVERALL VOLUME

No matter how good your guitar pickup is, if the overall volume in the room is high
enough, your guitar may generate feedback. The box of your guitar is a resonant sound
chamber. The energy from the strings drive this box, but so do sound waves coming
from the room. Drums, other amplified instruments, screaming fans, all generate sound
frequencies that can resonate within your guitar, and be amplified through your pickup.

(This is one of the advantages of semi-hollow and solid body guitars- they reduce the
impact of ambient sound on the guitar pickups by stiffening the resonant chamber. The
down side of course is that they don’t sound as nice un-amplified. On the up side, when
the impact of resonance on the guitar body is out of the picture you can actually use
feedback as an effect ! Controlled feedback, though, just isn’t embraced as much by
acoustic players.)

The bottom line is that, with most acoustic guitar pickups, keeping the overall volume of
all the instruments in the room as low as possible helps reduce the potential for feedback.

PRE-AMP SETTINGS

In my opinion, anyone with a pickup in their acoustic guitar should have a pre-amp,
either mounted on the guitar or as an outboard unit between the guitar and the amp. Wise
use of the volume and tone controls on your pre-amp can help to reduce feedback.

Here is some advice:

1. Set the volume at the pre-amp as low as possible and get most of your volume
from the amplifying system. This makes your pickup less sensitive to the overall
volume in the room.

2. Use the tone settings on your pre-amp to control feedback. Every guitar has a
harmonic resonance point. This resonance point will vary depending on the size of the
sound chamber and the stiffness of the construction. On a given guitar, increasing the
volume in this tonal range can generate feedback. Conversely, reducing the volume in
this specific tonal range enables increasing the volume of the remaining tonal spectrum.
So, if volume adjustment alone isn’t enough to stop feedback, experiment with your tone
settings. Often, dropping mid-range frequencies is a good place to start. Also, a given
room will react, sonically, in its own way; so you may have to experiment with these
settings each time you play in a different venue.

MECHANICAL ISSUES WITH THE GUITAR

Both the structure of your guitar and the correct installation of the pickup can impact its
performance when amplified.

GUITAR STRUCTURE

At higher volumes a guitar box with light construction may react more to ambient sound
waves, which can lead to feedback issues in guitars with soundboard transducers. From
the point of view of feedback resistance, guitars with light construction may be better
served by under-saddle type pickups since under-saddle pickups, contained as they are in
the wood of the bridge, are more isolated from the resonance of the guitar box. If
combination systems are installed, an under-saddle pickup or magnetic pickup should
always be included as an option. Unfortunately, it may not always be easy to make the
judgment about how likely a given guitar will be to react this way. In my experience I
have seen this issue more in small shop guitars because there is often
more attention paid to creating a responsive acoustic sound, which usually translates into a lighter structure, but this is not universal.

CORRECT INSTALLATION OF THE PICKUP

Pickups which are not installed correctly can lead to
feedback problems.

1. Under Saddle Pickups

For proper function, under-saddle pickups must be sandwiched evenly between the saddle
and the bottom of the slot in the wooden bridge. What this means is that, if the pickup
element were not in place, the bottom of the saddle would sit flush with the bottom of the
saddle slot. The pickup element is manufactured to very close tolerances, so if the saddle
and the saddle slot are true, there will be even pressure under each string and the string to
string volume should be appropriate. It is the down pressure from each string that
activates the pickup element. If there is any unevenness of contact between the saddle
and the bottom of the saddle slot in the bridge under a given string, it will not sound as
loud as the others. This unevenness may not lead directly to feedback issues, except when
increasing volume to compensate for a deadened string, but a well fitted saddle is
essential to proper pickup function.

A factor of the fit of the saddle which does play into feedback is the looseness of the
saddle in the slot. The saddle should not be floppy ”loose”, but it should be able to slide
up and down without pinching. A common feedback issue with under saddle pickups
results from the saddle not fully pressing down on the pickup element because it is
slightly wider than the slot. The bottom line is that a good fit of the string saddle is
necessary to avoid feedback issues.

2. Contact Pickups

There are two factors which commonly affect the quality of sound, and resistance to
feedback of contact pickups- location and quality of adhesion.

Most contact pickups currently being manufactured are designed to be installed inside
the top directly under the saddle position. This location affords proximity to the focus of
the string energy as well as being a fairly stiff place on the top. It should be noted that
some contact pickups, as in the Taylor “Expression” system, which combines contact and
magnetic pickups, are located on other parts of the top. But most contact pickups which
are retrofitted to an instrument will best respond, and resist feedback, when mounted in
the area under the bridge.

Contact pickups are generally held in place with either industrial-strength two sided tape,
acoustic putty, or super glue. Regardless of the material, the bond to the top must be firm.

3. Microphones

Microphones are either clipped to a brace, attached to a flexible gooseneck, mounted to
the pre-amp, or held in place with an adhesive Velcro tab. For a given mike in a given
guitar there may only be one “sweet” spot- a place where the sound is reasonably pure
and somewhat resistant to feedback. The only way to find this spot is to experiment with
locations until you like the results. In high volume situations microphones are the most
prone to feedback issues and often cannot be used.

ADDITONAL SOLUTIONS

MAGNETIC PICKUPS

Reducing the overall sound coming into the box of the acoustic guitar, and wise use of the volume and tone controls
of your pre-amp will help reduce the potential
for feedback issues, but if you have no other choice, magnetic pickups, while more
commonly used in “electric” guitars, have their place in acoustic guitars as well. They are
usually found as units that clamp into the soundhole. Some companies, most notably
Taylor guitars (though Gibson did it back in the 1960’s), have incorporated magnetic
pickups into their factory installed pickup systems, usually placing them in or near the end
of the fretboard. It should be noted that, while magnetic pickups are rightly appreciated for their ability to largely
alleviate the problem of feedback when working in high volume situations, even the best
magnetic pickups don’t get high marks for their ability to reproduce the “natural” sound
of the guitar. By definition, a magnetic pickup is largely, though not entirely, triggered by
the vibration of the metal strings within the magnetic field generated by the pickup. Also,
pickups which are clamped into the soundhole both stiffen the guitar top, and, maybe
even more importantly, block the soundhole. Therefore, if you use a magnetic soundhole
pickup, you may want to have it installed such that you can remove it when not in use.

I would also say that a magnetic pickup, combined with a quality digital
processing system can create a wonderful acoustic “sound”, but, and I know we can get
into a semantic argument here, many people don’t consider a digitally produced guitar
sound to be “real”. (For the purpose of this article I am going to consider analog
electronic sound reproduction as “real” acoustic sound, though that point can also be
debated).

SOUNDHOLE COVERS

There is one product designed to help reduce feedback which I will mention because,
well, it is just a great invention. It is made by the Kaman Co. and is called the “Feedback
Buster” (part #FBR2). It is a round rubber disc that fits in the soundhole of the guitar. It is
made in only one size- 4” diameter. This will fit most Martin dreadnaughts and many
others, but not all guitars. Also, it cannot be used if you have soundhole mounted volume
controls.

The concept is simple: It blocks all external sound from coming through the soundhole
which, in turn, markedly reduces the potential for feedback in high volume situations. I
have also seen a “lute” style soundhole cover advertised which also seems to address this
same issue. I can attest to the benefit of soundhole covers, and do not hesitate to use them
when everything else about the guitar’s sound system has been addressed and feedback is
still an issue.

THE BOTTOM LINE

In summation- to reduce the chance for feedback when amplifying your acoustic guitar: don’t point speakers at your guitar, try to keep the
overall volume of all the instruments from being too loud, use the volume and tone
controls on your pre-amp to your advantage, make sure your guitar is set up properly and
the pickup installed correctly, and, if necessary, cover the soundhole. Good luck!